The invention generally relates to implantable medical devices, such as pacemakers or implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d) and to external programmers for use therewith and, in particular, to techniques for emulating a surface electrocardiogram (EKG) based on internal electrical cardiac signals.
A pacemaker is a medical device, typically implanted within a patient, which recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is a device, also implantable into a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation. Pacemakers and ICDs detect arrhythmias by sensing internal electrical cardiac signals using leads mounted within the heart. The internal signals comprise an intracardiac electrogram (IEGM). More specifically, the normal contraction of atrial heart muscle tissue appears as a P-wave within the IEGM. A sequence of consecutive P-waves defines the atrial rate. The normal contraction of ventricular muscle tissue appears as an R-wave (sometimes referred to as the xe2x80x9cQRS complexxe2x80x9d) within the IEGM.
Pacemakers and ICDs are often configured to be used in conjunction with a programmer that allows a physician to program the operation of the implanted device to, for example, control the specific parameters by which the device detects arrhythmia conditions and responds thereto. For example, the programmer may allow the physician to specify the sensitivity with which the implanted device senses electrical signals within the heart and to further specify the amount of electrical energy to be employed for pacing the heart in circumstances where expected heart signals are not sensed. Additionally, the programmer may be configured to receive and display a wide variety of diagnostic information detected by the implanted device, such as graphs of the IEGM sensed by the implanted device. In addition, the programmer may operate to analyze the data received from the device to assist the physician in rendering diagnoses as to possible arrhythmias and to assist the physician in programming the device to provide appropriate therapy.
Current state of the art implantable cardiac stimulation devices may have dozens or hundreds of programmable parameters that can be individually programmed using the external programmer. The programmable parameters permit the operation of the cardiac stimulation device to be tailored to the needs of the particular patient to provide optimal therapy while minimizing the risk of any unnecessary therapy. Unfortunately, it is often difficult to predict what the resultant operation will be for any given patient with any selected set of parameter settings. Hence, a potentially viable set of parameters is chosen by the physician, the implantable cardiac stimulation device is programmed using the selected set of parameters, and then the patient is sent home. Weeks or months later the patient must return to the physician""s office for a follow-up appointment so that they physician may evaluate the results of the selected parameters.
Typically, the follow-up evaluation consists of the physician making judgments based upon a review of diagnostic information provided by the implanted device (including the IEGM) in combination with a 12-lead surface electrocardiogram (EKG or ECG) provided by a separate surface EKG unit. The 12-lead EKG unit provides twelve separate signals (detected using electrodes attached to different locations on the patient) that can be individually processed, displayed and reviewed or can be combined to yield a single combined surface EKG. As part of the review, the physician often needs to compare new 12-lead surface EKGs with recorded 12-lead surface EKGs from previous sessions. In any case, the physician adjusts the programming of the implanted device to improve therapy delivered to the patient. Again, the patient is sent home for several more weeks or months until another follow-up visit. This cycle may be repeated numerous times before optimal device settings are determined by the physician.
To obtain a 12-lead surface EKG, ten electrodes are manually attached the skin of the patient in the configuration shown in FIG. 1. The surface EKG derived from the ten electrodes is referred to as a xe2x80x9c12-leadxe2x80x9d EKG because twelve signals are derived from the ten electrodesxe2x80x94including signals from individual electrodes plus signals between certain pairs of the physical leads. More specifically, the ten electrodes include four limb electrodes and six xe2x80x9cchestxe2x80x9d electrodes. The chest electrodes are labeled: V1-V6. The limb electrodes are: RA (right arm), LA (left arm), LL (left leg) and right leg (RL), the last of which is optional The chest electrodes provide one signal per electrode, referred to as the V1-V6signals. The RA, LA and LL limb electrodes also provide one signal per electrode, referred to as the aVR, aVL and aVF signals (with F signifying foot as opposed to leg.) Finally, the difference between each pairing of the RA, LA and LL limb electrodes is considered a separate xe2x80x9cleadxe2x80x9d (referred to as the Einthoven leads I, II and III) and hence provide the last three signals of the 12-lead surface EKG. The twelve signals of the 12-lead surface EKG are summarized in TABLE I, along with the electrodes from which the signals a rederived.
It is particularly important to review the 12-lead surface EKG during follow-up sessions. See: xe2x80x9cThe Paced 12-Lead Electrocardiogram Should No Longer Be Neglected in Pacemaker Follow-Upxe2x80x9d, by S. Serge Barold; Paul A. Levine; I. Eli Ovsyshcher, PACE 2001; 24:1455-1458. However, the need to manually attach and remove each of the surface EKG electrodes from the patient during each follow-up session is a burden to the physician (or his or her staff) and a considerable inconvenience to the patient. In many cases, the skin of the patient must be shaved and sanded in the locations where the electrodes are to be attached to provide adequate electrical conduction. This can be quite uncomfortable and, in some cases, embarrassing for the patient. Moreover, the time required to attach and then remove the electrodes adds to the overall cost of the follow-up session. Also, from one follow-up session and another, the electrodes may not be placed at the exact same locations on the patient, thus resulting in somewhat different surface EKGs and making it more difficult for the physician to properly identify any actual differences in cardiac signals of the patient from one session to the next.
As can be appreciated, it would be desirable to eliminate the need for attaching the electrodes of the 12-lead surface EKG to patients during follow-up sessions to thereby reduce the cost and inconvenience to the patient and to eliminate problems resulting form differing electrode placement. One proposed solution is to emulate the surface EKG using internal electrical cardiac signals sensed by the implanted device so that, during a follow-up session, a separate surface EKG system is not required and external electrodes need not be attached to the patient. One technique for emulating a surface EKG using internal electrical signals is described in U.S. Pat. No. 5,740,811 to Hedberg et al., entitled xe2x80x9cDevice and Method for Generating a Synthesized ECGxe2x80x9d, which is incorporated by reference herein. With the technique of Hedberg et al., a neural network is employed to convert electrical signals derived from implanted electrodes into a single emulated or xe2x80x9csynthesizedxe2x80x9d surface EKG.
The technique of Hedberg et al. achieves significant improvement by eliminating the need to use a separate surface EKG unit during follow-up sessions. However, there is considerable room for further improvement. The neural network technique of Hedberg et al. appears to emulate only a single combined EKG and does not emulate each of the twelve individual signals of the conventional 12-lead EKG. Hence, the physician cannot review the individual signals nor use any hardware or software adapted for separately processing the individual signals. Accordingly, it would be desirable to provide a surface EKG emulation technique that separately emulates each of the twelve signals of a conventional 12-lead EKG and it is to this end that aspects of the invention are directed.
In addition, the technique of Hedberg et al. does not appear to take into account factors affecting the relative locations of implanted electrodes during the emulation. In particular, respiration, posture and the beating of the various chambers of the heart all affect the relative locations of internal electrodesxe2x80x94both with respect to one another and with respect to the locations of surface electrodes of the EKG being emulatedxe2x80x94and hence affect the accuracy of EKG emulation. Respiration causes the heart to twist slightly thus changing the relative locations of electrodes mounted within the heart, particularly with respect to the location of the device can. The beating of the various chambers of the heart during different phases of a cardiac cycle also change the relative locations of the electrodes. Differences in overall patient posture (i.e. whether the patient is sitting, standing, or lying down) also affect the location of the device can and the location of the heart and another internal organs and hence affect the relative locations of the internal electrodes. Without taking these factors into account, precise EKG emulation is not achieved. Accordingly, it would be desirable to provide a surface EKG emulation technique that takes into account factors affecting the relative locations of internal electrodes and it is to this end that aspects of the invention are directed.
Still other aspects of the invention are directed to providing a calibration technique for calibrating the surface EKG emulation for use with a particular patient and a verification technique for automatically verifying the reliability of the surface EKG emulation. Any significant change in the reliability of the surface EKG emulation is likely caused by lead dislodgment. Hence, the invention also provides a technique for automatically detecting possible lead dislodgment.
In accordance with one illustrative embodiment, a method is provided for emulating individual signals of a multiple-lead surface EKG of a patient in which an implantable cardiac stimulation device is implanted. The method includes inputting electrical cardiac signals sensed using a combination of pairs of electrodes implanted within the patient; and then emulating each of a plurality of separate signals associated with the multiple-lead surface EKG based on the input electrical cardiac signals.
For example, each of the twelve signals associated with a standard 12-lead surface EKG can be emulated, i.e. the technique provides for separate emulated V1, V2, V3, V4, V5, V6, I, II, III, aVR, aVL, and aVF signals.
The emulation technique may be performed by the implanted device itself or by an external device, such as an external programmer, configured to receive the electrical cardiac signals from the implanted device. In either case, by emulating each of the individual signals of the surface EKG, rather than merely generating a combined surface EKG signal, the separate signals can be individually processed using any of a wide variety of techniques, conventional or otherwise. For example, the individual signals can be filtered separately and displayed separately. All or just a selected portion of the individual signals can be combined, perhaps with the individual signals weighted differently, to generate a single surface EKG for actual display. Hence, a great deal more flexibility is achieved than with emulation systems that merely generate a single combined surface EKG.
Preferably, when the technique is performed by the implanted device, it does so only while in communication with the external programmer. In other words, the implanted device only emulates the surface EKG signals while in telemetry contact with the external programmer. Hence, the implanted device need only transmit the emulated surface EKG signals to the external programmer and so the emulated EKG signals need not be stored within the memory of the implanted device. However, in other implementations, the implanted device operates to continuously emulate the surface EKG at all times. If so, the emulated surface EKG is stored within the memory of the implanted device for subsequent transmission to the external programmer, perhaps during a follow-up session with the physician. In this manner, a diagnostic record of the emulated surface EKG of the patient is recorded within the implanted device (limited only by the memory constraints of the implanted device) for subsequent review by the physician. In addition, the emulated surface EKG can be used in conjunction with an IEGM to aid in the control of delivery of therapy, such as to aid in identifying the onset of AF or VF. Hence, the technique of the invention is not limited to being performed by the implanted device only while in telemetry contact with the external programmer.
In an exemplary embodiment, emulation of the individual signals is 20 performed using a matrix-based technique. A combination of pairs of internal electrodes is selected and cardiac signals sensed using the selected pairs of electrodes are input. The input signals are converted into a time-varying vector F(t) having individual elements corresponding to the pairs of electrodes. Then, a conversion matrix M is input, which is composed of weighting factors representative of the relative extent to which the signals derived from the selected pairs of electrodes influence surface voltages at locations corresponding to the multiple-lead surface EKG being emulated. Finally, a time-varying vector E(t) is generated by calculating: E(t)=M*F(t). The individual time-varying elements of E(t) represent the individual emulated surface EKG signals. In an example wherein ten pairs of electrodes are selected to emulated a 12-lead surface EKG, vector F(t) has ten elements and conversion matrix M is a ten by twelve matrix thus providing an output vector E(t) with twelve elements corresponding to the twelve leads of the 12-lead surface EKG. In general, though, the selected combination of electrodes includes N pairs of electrodes and the surface EKG to be emulated includes M surface leads. F(t) has N elements, n=1, 2, . . . , N; E(t) has M elements, m=1, 2, . . . , M; and M is an N by M matrix containing weighting factors Knm representative of the extent to which the internal voltage derived from electrode pair n influences the surface voltage at surface lead m.
With the matrix-based technique, virtually any combination of pairs of internal electrodes can be used to emulate the surface EKG. Hence, the technique can be applied to a wide variety of implantable cardiac stimulation systems having a wide variety of lead arrangements. In additon, the technique can be used to emulate surface EKG signals for any number and combination of surface leads and locations, such as 10-lead EKGs, 6-lead EKGs, etc. Thus, again, great flexibility is provided. For a given internal lead arrangement, the combination of electrodes that achieves the most accurate surface EKG emulation is typically selected for use. In other cases, however, to gain a reduction in processing and memory resources, some other combination of electrode pairs is employed, which may not provide the highest degree of accuracy in the emulation.
In addition, in the exemplary embodiment, cross-correlation values of certain elements of vector F(t), which should not change significantly with time, are calculated and compared against baseline cross-correlation values to verify the reliability of the emulation. If the cross-correlation values differ significantly from the baseline values, the emulation is deemed to be unreliable. This may be caused by lead dislodgment. Hence, the invention also provides for automatic detection of possible lead dislodgment.
In accordance with a second aspect of the invention, a method is provided for emulating a multiple-lead surface EKG of a patient in which an implantable cardiac stimulation device is implanted wherein the emulation takes into account factors affecting the relative locations of leads implanted within the patient such as respiration, posture and phase of cardiac cycle. The method includes the steps of: inputting electrical cardiac signals sensed using a combination of pairs of implanted electrodes; inputting a signal representative of factors affecting the positions of the electrodes; and generating an emulated surface EKG based on the electrical cardiac signals while taking into account the factors affecting the positions of the electrodes. By taking into account such factors as respiration, phase of cardiac cycle, and posture, a more accurate emulation of the surface EKG is achieved that compensates for movement of the internal electrodes relative to one another and relative to the lead locations of the surface EKG being emulated.
In the exemplary matrix-based implementation, slightly different separate conversion matrices are used depending upon the factors affecting the relative locations of the internal electrodes. Alternatively, the weighting factors Knm of a single conversion matrix M are adjusted. If multiple factors affecting relative lead location are to be compensated for simultaneously, then either the separate conversion matrices are averaged together to yield a single adjusted matrix M or the weighting values of the single matrix M are adjusted. In any case, by compensating for movement of the internal electrodes, a far more accurate and reliable emulation of the surface EKG is achieved. Indeed, the emulated surface EKG may be more reliable than an actual surface EKG. As noted above, variation in placement of the surface electrodes on the skin of the patient from one session to another can result in unintended variations in the surface EKG. By instead generating a surface EKG using internal signals and by compensating for the relative movement of the internal leads, a more accurate, reliable and consistent surface EKG is thereby generated.
In accordance with a third aspect of the invention, a calibration or set-up method is provided for calibrating a surface EKG emulation technique. The calibration method is performed by external programmer in combination with an implantable cardiac stimulation device implanted within a patient and a multiple-lead surface EKG unit. Separate initial multiple-lead surface EKG signals are input to the external programmer from the EKG unit as detected using separate surface electrodes attached to the patient. Initial internal electrical cardiac signals sensed by the implanted device using internal electrodes are also input to the external programmer. A set of conversion values for converting internal cardiac signals into separate multiple-lead surface EKG signals are then generated, based on a comparison of the initial surface EKG signals and the initial internal cardiac signals. Then, separate multiple-lead surface EKG signals are emulated based on newly sensed internal electrical cardiac signals using the conversion values. Once the conversion values have been generated, actual emulation of the surface EKG signals may be performed either by the external programmer or by the implanted device. In any case, with this set-up technique, the emulation process is thereby calibrated for use with a particular patient based on surface EKG signals detected for that patient.
When using the exemplary matrix-based implementation, the set of conversion values is generated as follows. The initial internal cardiac electrical signals are converted into a time-varying vector F(t) having individual elements corresponding to the various pairs of electrodes. The initial surface EKG signals are converted into a time-varying vector E(t) having individual elements corresponding to the surface leads A time-varying conversion matrix M(t) of weighting factors is generated by calculating M(t)=E(t)*Fxe2x88x921(t). The time-varying conversion matrix M(t) is averaged over time to yield a fixed matrix M for use in converting newly-sensed internal cardiac signals into surface EKG signals. Thus, this provides a technique for generating an individual conversion matrix M. As noted, multiple conversion matrices may be used depending upon the implementation. For example, if factors affecting the relative locations of the internal electrodes are to be subsequently detected and compensated for during EKG emulation, then during the setup process either separate individual conversion matrices are generated and stored using this technique or a single conversion matrix is generated and then values for adjusting the weighting values Knm of the single conversion matrix are generated and stored.
Thus, in its various embodiments, the invention provides for emulation of the separate signals of surface EKGs while taking into account factors affecting the relative locations of internal electrodes. The invention also provides a calibration technique for calibrating the surface EKG emulation for use with a particular patient and a verification technique for automatically verifying the reliability of the surface EKG emulation. The invention also provides a technique for automatically detecting possible lead dislodgment. Other objects, features and advantages of the invention will be apparent from the descriptions below in combination with the accompanying drawings.